1. Field of the Invention
The present invention relates generally to a long haul optical waveguide fiber, and particularly to such an optical waveguide fiber that has negative dispersion. The waveguide fiber of the invention can be cabled and used to form all or a portion of an optical telecommunications link.
2. Technical Background
Communications systems operating at bit rates above about a giga-hertz or which include wavelength division multiplexing are facilitated through use of high performance waveguides. In such high performance systems launched power can range from 0.1 mW to 10 mW and higher. In the higher power systems, the desired properties of the waveguide fiber include larger effective area. New system strategies are being sought to decrease cost even while system performance is being enhanced.
A promising strategy is one that involves matching system components in such a way that a particular property of one component compensates a drawback in another component. Preferably, the component matching strategy is one in which a given component is designed to allow another component to operate more efficiently or effectively. Such compensation schemes have been effective, for example, in reducing dispersion penalty by adding a dispersion compensating module to within a communications link, thereby providing for a desired signal to noise ratio or signal pulse shape after the signal pulse has traversed the optical waveguide fiber of the link. Another example of effective compensation is the use of large effective area waveguide fiber in communications systems in which non-linear effects are a major source of signal degradation.
One area which can provide an increase in performance and a decrease in cost is that of matching a signal source to a fiber. A cost effective signal source, having relatively high power output and good longevity is the distributed feedback laser (DFB) which is directly modulated. However a directly modulated DFB laser is always positively chirped. That is, the leading edge of the pulse is shifted to longer wavelengths (red shifted) and the trailing edge is blue shifted. When such a pulse propagates in a positive dispersion fiber, the positive chirp results in pulse broadening. Efforts have been made to reduce the effect of positive chirp by biasing the semi-conductor laser above threshold. See Fiber Optic Communications Systems, G. P. Agrawal, p. 223.
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments. The definitions of the radii used herein are set forth in the figures and the discussion thereof.
Total dispersion of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-mode dispersion is zero.
The sign convention generally applied to the total dispersion is as follows. Total dispersion is said to be positive if shorter wavelength signals travel faster than longer wavelength signals in the waveguide. Conversely, in a negative total dispersion waveguide, signals of longer wavelength travel faster.
A chirped laser is one that produces an output pulse wherein the wavelengths within the pulse wavelength are shifted backward or forward in time. That is, the output pulse is red or blue shifted. A laser having a positive chirp is one in which the leading edge of the output pulse is red shifted and the trailing edge blue shifted.
The effective area is
Aeff=2xcfx80(∫E2rdr)2/(∫E4rdr),
xe2x80x83where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide. An effective diameter, Deff, may be defined as,
Aeff=xcfx80(Deff/2)2.
The relative refractive index percent, xcex94%=100xc3x97(ni2xe2x88x92nc2)/2ni2, where ni is the maximum refractive index in region i, unless otherwise specified, and nc is the average refractive index of the cladding region.
The term xcex1-profile refers to a refractive index profile, expressed in terms of xcex94 (b)%, where b is radius, which follows the equation,
xcex94(b)%=xcex94(bo)(1xe2x88x92[¦bxe2x88x92bo¦/(b1xe2x88x92bo)]xcex1),
xe2x80x83where bo is the point at which xcex94(b)% is maximum, b1 is the point at which xcex94(b)% is zero, and b is in the range bixe2x89xa6bxe2x89xa6bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and a is an exponent which is a real number. The initial and final points of the xcex1-profile are selected and entered into the computer model. As used herein, if an xcex1-profile is preceded by a step index profile or any other profile shape, the beginning point of the xcex1-profile is the intersection of the xcex1-profile and the step or other profile.
In the model, in order to bring about a smooth joining of the xcex1-profile with the profile of the adjacent profile segment, the equation is rewritten as;
xcex94(b)%=xcex94(ba)+[xcex94(bo)xe2x88x92xcex94(ba)]{(1xe2x88x92[¦bxe2x88x92bo/(b1xe2x88x92bo)]xcex1},
where ba is the first point of an adjacent segment.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two measured attenuations. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. (The market code #70 mesh is descriptive of screen made of wire having a diameter of 0.178 mm. The screen openings are squares of side length 0.185 mm.) A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation in dB/m is measured. This increase in attenuation is the lateral load attenuation of the waveguide.
Adiabatic chirp is proportional to the output power of the signal.
Transient chirp is proportional to the derivative of the output power of the signal and so is present only in the time periods when the signal power is in transition between a 0 and a 1 (or a 1 to a 0).
Gain compression factor, also known as the nonlinear gain parameter, refers to a semiconductor laser and is a proportionality constant that relates semiconductor laser material optical gain of the active region of the laser to the number of photons in the active region. In the relationship, G=f(xcex5P), G is the gain of the laser, xcex5 is the gain compression factor, P is number of photons in the active region (which is directly related to the laser output power) and f is a function. See Fiber Optic Communications Systems 2nd Edition, Agrawal, page 113.
A more effective approach to optimize the performance of a positively chirped laser makes use of waveguide fiber having a negative total dispersion. In this case, the positively chirped pulse propagating in a negative total dispersion fiber undergoes compression and hence a negative dispersion power penalty is obtained. Applicants have discovered that by using the fiber disclosed herein, which has negative dispersion at the desired operating wavelength, together with a positively chirped laser, very long link lengths can be achieved before regeneration is needed. For example, relative to standard single mode fiber, a factor of three increase in fiber transmission distance is achievable using the negative dispersion fiber together with a positively chirped laser in accordance with the invention.
One embodiment of the present invention is an optical waveguide fiber having a core region surrounded by and in contact with a clad layer. The core region has at least two segments. Each of the segments is characterized by respective refractive index profiles, inner and outer radii, and relative index percents, all of which are preferably positive relative to the average value of core refractive index as defined above. The segment refractive index profiles are preferably selected to provide a waveguide fiber having negative total dispersion at 1530 nm and a positive total dispersion slope over a wavelength range of, 1525 nm to 1625 nm, which covers both the C and L wavelength bands. The zero dispersion wavelength is preferably greater than 1560 nm, more preferably greater than 1580 nm, even more preferably greater than 1610 nm, and most preferably greater than 1625 nm. The optical waveguide of the invention can be made to operate over other wavelength ranges by adjusting the refractive index profile of the core region such that for any given operating wavelength range the zero dispersion wavelength is greater than or equal to the longest wavelength of that range. The waveguide is preferably single mode over a selected operating wavelength range, although operation in a wavelength range where the waveguide propagates two or more modes is possible. In some cases, a waveguide that propagates two or more modes is effectively single mode fiber because the higher order modes are strongly attenuated so that these higher modes disappear before reaching the receiver end of a communication system.
The fibers of the present invention have applicability in the 1310 nm operating window (1250 nm to 1370 nm), the 1550 nm operating window (1530 nm-1565 nm), the L-band (1565 nm-1625 nm), and higher wavelengths up to 1700 nm. In other words, the fibers disclosed herein can potentially be utilized at any wavelength between 1250 nm and 1700 nm.
In another embodiment, the present invention is a waveguide fiber telecommunications link, operating over a desired operating wavelength range, including a laser light source, a receiver, optically coupled to each other by at least one waveguide fiber made in accordance with the invention. The laser light source is positively chirped. The at least one waveguide fiber of the link has negative total dispersion at 1550 nm. A positively chirped pulse from the laser will be compressed upon entering the negative dispersion waveguide fiber. That is, the negative dispersion waveguide fiber produces a blue shift in the leading edge of wavelengths within the pulse that offsets the red shift of the wavelengths in the pulse caused by the positive chirp. The negative dispersion fiber also red shifts the trailing edge of wavelengths within the pulse. The result is that the pulse is compressed, thereby providing negative dispersion power penalty. It is to be understood that the waveguide fibers in a link are typically cabled or otherwise buffered.
Laser chirp can be characterized as adiabatic which means the chirp is proportional to the optical output power of the laser. In contrast, transient chirp is proportional to the rate of change of optical output power with time. In the case of the directly modulated DFB lasers, the chirp is predominantly adiabatic when the laser is always operated well above threshold with low extinction ratios (e.g. 6 dB). Furthermore, adiabatic laser chirp is characterized by a relatively high gain compression factor, e.g., one in the range of 4xc3x9710xe2x88x9223 m3 to 30xc3x9710xe2x88x9223 M3.
However with present technology the chirp becomes predominantly transient when the laser is operated closer to threshold, where the extinction ratios becomes much higher (e.g. 12 dB). The exact extinction ratio or drive condition under which a laser""s chirp switches from predominantly adiabatic to predominantly transient depends upon the exact parameters of the laser itself. In the case of a transient chirp operation of a DFB laser the gain compression factor in transient operation is no greater than about 1xc3x9710xe2x88x9223 m3. Gain compression factor for a particular laser structure may be measured by using fitting techniques described, for example, in L. A. Coldren and S. W. Corzine, xe2x80x9cDiode lasers and photonic integrated circuitsxe2x80x9d, Wiley, 1995, p.211, xe2x80x98Intensity modulation and chirp of 1.55 um MQW laser diodes: modeling and experimental verificationxe2x80x99, K. Czotscher et. al., IEEE Journal of Selected Topics in Quantum Electronics, vol. 5, no. 3, May/June 1999, or, xe2x80x98Extraction of DFB laser rate equation parameters for system simulation purposesxe2x80x99, J. C. Cartledge et. al., IEEE Journal of Lightwave Technology, vol.15, no. 5, May 1997.
Recent work indicates that transient chirp dominated laser operation is preferred. The eye diagram remains open over a longer fiber distance in the case of transient operation.
While the preferred operating wavelength of the link is 1530 nm to 1565 nm, a more preferred operating range is 1530 nm to 1625 nm, an even more preferred operating range is 1250 nm to 1625 nm, and a most preferred operating range is 1250 nm to 1700 nm.
The telecommunications link of the invention can include only waveguides having negative dispersion or can include waveguide having positive dispersion. The link can include both positive and negative dispersion waveguides. The positive and negative dispersions of the waveguides forming a link of this type are preferably selected so that the positive dispersion of the positive dispersion waveguides substantially cancels the residual negative dispersion of the negative dispersion waveguides. The residual negative dispersion of the negative dispersion waveguides is the portion of the negative dispersion that is not effectively canceled by the positive chirp of the light source. The waveguides making up the link can be selected to provide an end to end link dispersion that is positive, negative, or zero. In one embodiment the magnitude of the end to end dispersion of the link is less than 10 ps/nm and preferably less than 5 ps/nm at 1550 nm. Of particular value is a link in which end to end dispersion is very small, to provide minimal signal distortion, while the respective dispersions of the waveguides forming the link are not too close to zero (not less than about 0.05 ps/nm-km) so that dispersion penalty due to non-linear four wave mixing is avoided.
The waveguide fiber forming the link generally is cabled or otherwise protected.
Yet another aspect of the invention is a dispersion compensated communications link in which the dispersion compensating waveguide fiber is a negative dispersion waveguide in accordance with the first aspect of the invention set forth above. The compensating waveguide fiber has attenuation at an operating wavelength less than about 0.5 dB/km, preferably less than 0.25 dB/km, and more preferably less than 0.22 dB/km attenuation at a selected wavelength. For example, attenuation at 1550 nm is preferably less than 0.25 dB/km, and more preferably less than 0.22 dB/km. The compensating waveguide fiber is at least comparable to standard step index single mode fiber in resistance to bending loss so that the compensating fiber can be cabled or otherwise protected and comprises a portion of the link length. The relatively lower effective area of the negative dispersion waveguide compensating fiber can be reduced as a source of non-linear power dispersion in a communications link by placing the negative dispersion fiber away from the signal transmitter. This link configuration ensures the signal will be lower in amplitude when traveling in the lower effective area compensating waveguide, thereby minimizing non-linear dispersion effects.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.